Organic Chemistry Chapter 10
10.1 Homologous Series
Overview
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Organic Chemistry Organic chemistry is the chemistry of carbon containing compounds. From the very simple: methane To the very complex: Haem B
Homologous Series A homologous series is a family of compounds that differs only by the length of its hydrocarbon chain Members share: General formula Chemical properties Three such series are the: Alkanes -single bonds Alkenes -double bond Alcohols (-OH)
Homologous Series and Boiling Points What do you think will be the trend in melting/boiling points as you go down a homologous series? Why?
Formulas Draw the compound with the formula C 4 H 8 O
What did you get? Clearly a molecular formula is not enough!
Types of Formula Empirical Formula C 4 H 8 O C 4 H 8 O Molecular Formula C 4 H 8 O C 4 H 8 O Full Structural Formula Aka displayed formula Condensed Structural Formula Note the = used for the C=C double bond CH 2 =CHCH 2 CH 2 OH CH 2 =C(CH 3 )CH 2 OH Skeletal formula Not required but v. useful Used in data booklet for complicated structures Do not use in exam answers!
Key Points Organic chemistry is the chemistry of carbon containing compounds A homologous series is a family of organic compounds differing only by the length of their carbon chains The melting and boiling point increases as you go down a homologous series Displayed formulas show the unambiguous arrangement of atoms in a compound
Isomers
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Isomers Compounds with the same molecular formula but different structural formula The 20 different C 4 H 8 O compounds from last lesson are isomers of each other These are all structural isomers Same number of each atom, but bonded in a different order You would have even more if you included geometric and optical isomers
Structural Isomers of the Alkanes The (non-cyclic) alkanes have the general formula C n H 2n+2 Draw full and condensed structural formulas for every isomer of every one of the alkanes up to n = 6 If you finish early, draw each as a skeletal formula
Did you get them all?
And skeletally
Naming Straight-chain alkanes Suffix: Tells us the functional group of the molecule For alkanes it is -ane Prefix: Tells us the length of the longest carbon chain: 1 carbon: meth- 2 carbons: eth- 3 carbons: prop- 4 carbons: but- 5 carbons: pent- 6 carbons: hex- Example 1: ethane Example 2: butane: Task: write in the names of the 4 straight chain alkanes next to your diagrams from last slide
Naming branched-chain alkanes Start by naming the longest chain Add extras to say the size of a branch, its position and how many of that branch Example 1: 2- methylpropane Branch Size: 1 carbon: methyl- 2 carbons: ethyl- 3 carbons: propyl- Position: Number the carbons in the longest chain Choose numbers to minimise the total numbers used Example 2: 2,3- dimethylbutane Number of same branches One branch nothing Two branches di- Three branches tri- Four branches tetra- Task: name the remaining alkanes
The straight-chain alkenes Alkenes are the same as alkanes but have one C=C double bond. The suffix for the alkene homologous series is -ene Task: draw full structural and skeletal formulas for each of the straight-chain alkenes up to C6 and name them. Do the branched ones as well if you have time Hint: you need to state the position of the double bond, but only if there is the possibility of multiple isomers: i.e. but-2-ene or hex-1-ene but only ethene not eth-1-
Did you get them?
Key Points Structural isomers have the same number of each atom but they are connected differently When naming compounds The longest carbon chain forms the prefix The functional group tells you the suffix Sometimes numbers need to be used to tell you where this functional group is Side chains and other groups are named according to what they are, how many there are and their position
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Key Points There are 7 functional groups we need to know in detail See functional group handout
Chapter 10.2 Alkanes
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Combustion of Alkanes (5 mins) The alkanes really don t do much Combustion is of one of two notable reactions (this is why we use them for fuels) Complete combustion: alkane + oxygen carbon dioxide + water Incomplete combustion: Alkane + oxygen carbon + carbon monoxide + carbon dioxide + water The amounts of C, CO and CO 2 will vary depending on conditions Task: Observe the combustion of the gas from the gas taps (propane/butane mix) and of a small amount hexane (in spirit burners). Hold the end of a clean boiling tube just over the flame for 15 seconds, this will collect soot from the flame. Record all observations clearly and try to account for them Include balanced equations to describe the (complete) combustion
Halogenation Alkanes will undergo halogenation if reacted with a halide in the presence of u.v. light. For example: C 2 H 6 (g) + Cl 2 (g) ethane u.v. CH 3 CH 2 Cl(g) + HCl(g) chloroethane This reaction is an example of free radical substitution
Radicals Radicals are species with unpaired electrons They are crazy reactive Halogens form radicals when hit by uv light of the right frequency: u.v. Cl 2 2 Cl The dot after the Cl represents the unpaired electron and tells us we have a radical This process is called homolytic fission the bond breaks equally with one electron going to each chlorine Task: draw Lewis structures for the Cl 2 molecule and each of the Cl radicals
Reaction Mechanism: Free Radical Substitution u.v. Cl 2 2 Cl Initiation Radicals formed by homolytic fission Propagation Cl + C 2 H 6 C 2 H 5 + HCl C 2 H 5 + Cl 2 C 2 H 5 Cl + Cl These steps feed each other the radicals needed to continue Cl + Cl Cl 2 Cl + C 2 H 5 C 2 H 5 Cl C 2 H 5 + C 2 H 5 C 4 H 10 Termination Any two radicals can combine to terminate the reaction Concentration of radicals is low so this is a rare event A single radical can cause thousands of cycles of the propagation stage before it reaches termination This same mechanism applies to all of the halogens The alkane can be substituted multiple times, until every H has been replaced
Key Points Alkanes are pretty unreactive They release a lot of energy on combustion, and are easy to handle which makes them good fuels Undergo free radical substitution to form halogenoalkanes and a hydrogen halide in the presence of UV light
Alkenes
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Reactivity of Alkenes Alkenes are considerably more reactive than alkanes and are a major industrial feedstock The reactivity is due to the double bond: The double bond contains 4 electrons This is a significant amount of charge which: Makes it attractive to electrophiles Enables it to polarise approaching molecules Most reactions of alkenes are addition reactions where two molecules come together to make one new one
Alkenes and hydrogen Alkene + hydrogen alkane Reaction conditions: Hot Ni catalyst This is an addition reaction, in which the hydrogen adds across the double bond
Alkenes and hydrogen halides Alkene + hydrogen halide halogenoalkane Reaction conditions: This reaction occurs very readily and needs no special conditions This is an addition reaction, in which the hydrogen halide adds across the double bond
Alkenes and halogens Alkene + halogen dihalogenoalkane Reaction conditions: This reaction occurs very readily and needs no special conditions If the halogen used is an aqueous solution of bromine (bromine water), the orange-brown colour of bromine solution is decolourised. This is the standard test for alkenes.
Alkenes and water Alkene + water alcohol Reaction conditions: Water must be steam Phosphoric or sulphuric acid catalyst This is the process used to make industrial ethanol Fermentation from sugar would be far too expensive!
Polymerisation Under the right conditions, alkene molecules will add to each other creating a polymer In this case, 1-bromo-2-fluoroethene polymerises to form poly- 1-bromo-2-fluroethene Conditions: Vary from alkene to alkene but often include high pressure, temperature and a catalyst The carbons in the C=C double bonds form the carbon chain, everything else hangs off this chain
Drawing polymers Draw three-monomer lengths of the polymers formed by: Propene Styrene Pent-2-ene
Key Points Alkenes undergo addition reactions with: Hydrogen Hydrogen halides Halogens Water (steam) Alkenes undergo addition polymerisation Alkenes are very economically important due to the range of products they can make
Alcohols
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Alcohols as Fuels Alcohols combust more readily than equivalent alkanes but release less energy since they are already partially oxidised Alcohol + oxygen carbon dioxide + water Alcohols are used as fuels: As a fuel for cars either pure or blended with petrol Methanol as fuel for competitive motorsports including dragsters and monster trucks Much fuel ethanol is fermented from crops crops that could otherwise be eaten, forcing up food prices. Is this ok?
Oxidation of alcohols The most important reactions of the alcohols are their oxidations A range of compounds will oxidise them so the oxidiser is often represented as [O] One oxidising agent you need to know is potassium dichromate, K 2 Cr 2 O 7. When using this, orange Cr (VI) is reduced to green Cr (III) More on what this means in the oxidation and reduction unit See next slide for details
Oxidation reaction scheme
Key Points Alcohols are highly combustible Primary alcohols oxidise to form aldehydes, which oxidise to form carboxylic acids Secondary alcohols oxidise to form ketones Tertiary do not oxidise due to the 3 strong C-C bonds surrounding the OH carbon
Halogenoalkanes
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Nucleophilic Substitution One of the most important reactions undergone by halogenoalkanes is nucleophilic substitution A nucleophile is a nucleus-loving species that is attracted to positive charges. Nucleophiles have either full negative charges or delta-negative charges Water and hydroxide are both nucleophiles In this case we can also call the reaction hydrolysis The carbon in the carbon-halogen bond has a δ+ charge due to the greater electronegativity of the halogen This makes it susceptible to attack by nucleophiles
Halogenoalkanes and strong bases A substitution reaction takes place, where the halogen atom is displaced by the hydroxide ion halogenoalkane + sodium hydroxide alcohol + sodium chloride Conditions: Aqueous base Gently warmed (can at room temperature, but may be quite slow) This is a nuclephilic substitution. The C attached to the halogen is δ + due to the high electronegativity of the halogen The OH - ion (our nucleophile) is attracted to the δ + carbon A nucleophile is a species with a negative charge or a lone pair that is attracted to positive/deltapositive atoms
S N 1 Unimolecular nucleophilic substitution animation here Unimolecular because only one molecule is involved in the rate determining step The rate determining step involves the spontaneous breaking of the carbon-halogen bond and is a heterolytic fission, forming a halide ion and a carbocation intermediate The stability of the carbocation intermediate is a key factor in S N 1 The attack by the nucleophile (OH - ) is very fast, but does need the carbocation to be formed first The rate is only dependent on the concentration of the halogenoalkane: Rate = k[halogenoalkane] Note: the curly arrows show the movement of pairs of electrons
S N 2 Bimolecular nucleophilic substitution animation here Bimolecular because two molecules are involved in the rate determining step In the rate determining step, the nucleophile (OH - ) attacks at the same time as the carbon-halogen bond breaks. The reaction passes through a negative transition state where the carbon has a half-bond to both the OH and the Br with an overall negative charge The rate is dependent on both the concentration of the halogenoalkane and the nucleophile Rate = k[halogenoalkane][nucleophile]
S N 1 or S N 2? 1 o halogenoalkanes predominantly undergo S N 2 2 o halogenoalkanes undergo a mix of S N 1 and S N 2 3 o halogenoalkanes predominantly undergo S N 1 You do not need to know why at SL, but will find out more at HL
Refresh Halogenoalkanes undergo substitution with strong bases to form alcohols The reaction has two possible mechanisms: S N 1: the C-X bond breaks and then the nucleophile attacks S N 2: the nucleophile attacks at the same time as the C-X bond breaks The mechanism depends on the halogenoalkane: 1 o - S N 2 2 o - S N 1 and S N 2 3 o - S N 1